1. Field of the Invention
This invention generally relates to a system for reducing controlled flight into terrain accidents and in particular it relates to terrain data processing and algorithms for terrain awareness and warning systems to prevent controlled flight into terrain accidents.
2. Brief Description of the Prior Art
Controlled Flight into Terrain (CFIT) warning uses the data provided by a flight management computer, Global Positioning System (GPS) receiver and other aircraft data providing systems. The prior art of CFIT warning predicts a three-dimensional flight path based upon a curve-fit extrapolation of the most recent position and velocity data received. This predicted flight path is then compared with the internal terrain map of the immediate area and an assessment of the potential threat of aircraft collision with terrain is computed. The type of systems described above could look up to 120 seconds in advance. This is the so called "time to clear" in contrast to "time before impact" warning, and also "terrain avoidance" in contrast to terrain "awareness".
The effort to reduce CFIT accidents by U.S. Airlines can be traced back to 1974. The prior art of standard Ground Proximity Warning System (GPWS) uses radio altimeter data to provide an audible warning if an aircraft has insufficient terrain clearance. Flying into precipitous terrain can result in late warnings since the standard GPWS depends upon a downward looking radio altimeter to detect rising terrain.
"Enhanced" version of GPWS-EGPWS, which is available from AlliedSignal, and Ground Collision Avoidance System (GCAS), which is available from Sextant Avionique, graphically depict terrain surrounding the aircraft's flight path on a cockpit display and provide earlier warning. Both systems are built around a three-dimensional terrain database and implement a true predictive look-ahead capability that is based on aircraft climb performance. The technology has three parts: use of GPS and other highly accurate navigation system to provide precise positioning with updates in seconds; computer technology with greater speed and memory; and accurate, sophisticated worldwide terrain databases including a Digital Terrain Elevation Database (DTED) of the world. FIG. 1 shows schematic system diagram of GPWS, which is from DASSAULT ELECTRONIQUE GCAS.
In EGPWS and GCAS, the terrain database is the system's core. Looking-ahead algorithms compare the projected future position of the aircraft with this database and warnings are issued on this basis. The using of DTED allows the systems to display terrain in proximity to the aircraft. During the flight, the peaks and terrain below the aircraft in DTED can also be displayed for situation awareness to the pilot. Looking ahead warnings based on comparison between flight profile or flight path and terrain can be used to give an advanced alert. During emergency descents in mountainous area and during en-route avoidance of inelement weather, looking-ahead warning further helps to prevent any situation that could lead to a CFIT.
However, the terrain database occupies large amounts of memory. If the terrain is mapped at 100-meter intervals over a region 10000 Km by 10000 Km, there are 10.sup.10 grid points. Clearly, the computations inherent in continuously accessing 10.sup.10 grid points are formidable and it is necessary to reduce, or compress this information. Besides, many navigation functions can be done with knowledge of terrain, such as allowing for optimum flight plan or emergency change of route in a real time situation, which could be restrained by the amount of data retrieves and the cost of computation when complexities of the algorithms are increased. Another major drawback of DTED file is that it only provides elevation data. Without further processing or aids from other formats of terrain data, such as features or vector representations, DTED gives no geometrical relationship among data elements.